Nanoceria (NC) has been shown to possess unique properties from its large scale complement such as the shifting and broadening of Raman-allowed modes [1], lattice expansion [2,3] and blue shift in ultraviolet absorption spectra [4]. As a result of these unique properties, NC has potential applications in UV protection, catalysis [5-9], and high-temperature oxidation resistance. Recently, it has been reported that small additions of lanthanides may confer even greater protection on those metals and alloys that are already well protected from corrosion by oxide films [10]. These include iron-chromium and iron-chromium-nickel stainless steels (i.e., both ferritic and austenitic alloys) and most other alloys that are dependent on chromium for their corrosion/oxidation resistance. Many high-temperature alloys rely on the formation of protective Al2O3 and Cr2O3 scales on their surfaces to resist high-temperature oxidation [10-13]. However, under various isothermal and thermal cycling conditions, these protective coatings crack due to thermal stresses and grain growth.
Oxide scale cracking and spalling restrict the application of such alloys as high-temperature oxidation resistant materials under demanding service conditions [10]. Addition of rare earth elements such as Ce, Y, Zr, La or their oxides improve the high-temperature oxidation resistance of alumina-and chromia-forming alloys due to the reactive-element effect (REE) [14-19]. Due to the REE, the oxide scale growth rate decreases, with an improvement in resistance to scale spalling as a result of increased scale-alloy adhesion.
Various researchers have put forward mechanisms to explain the REE. Antill and Peakall [11] indicated that the beneficial effect of the rare earth elements was primarily to improve scale plasticity for accommodating stresses due to the difference in the thermal expansion coefficients between the alloy and the oxide scale. The enhancement of oxide nucleation processes through the presence of rare earth elements was suggested by Stringer [12]. Tien and Pettit [13] reported that the application of rare earth elements provide sites for vacancy condensation in an Fe-25Cr-4Al alloy with consequent improvement of scale adhesion. A mechanism involving the pegging of the oxide scale to the alloy substrate has also been suggested [20]. Duffy and Tasker [21] supported the model of grain boundary blocking by Ce4+ ions, which associate with metal vacancies to form arrays of defect pairs along the grain boundaries. Moon and Bennett [22] concluded that the scale nucleates at the reactive-element oxide particles on the surface, blocks short-circuit diffusion paths by segregating reactive-element ions and reduces the stresses in the oxide scale by altering the microstructure.
It was first reported that ceria could be applied superficially rather than as an alloy addition and chromia growth could be slowed down in Ref. [23]. Earlier studies [24,25] indicated that superficial coating of micrometer-sized cerium oxide particles is effective in improving the high-temperature oxidation resistance of various grades of stainless steels (SS). Various researchers have carried out preliminary investigations on the improvement of the high-temperature oxidation resistance of Ni, Cr and Ni—Cr super alloys with the application of NC coatings [26,27]. It was also reported that NC coatings improve the high-temperature oxidation resistance of chromia forming steels [28]. However, detailed investigations into the effects of doped and undoped ceria nanoparticles and the role of oxygen vacancies in the improvement of high-temperature oxidation resistance of SS are yet to be carried out.